The insulin receptor is a tyrosine kinase which autophosphorylates itself and is capable of phosphorylating other substrates. Evidence strongly suggests that this characteristic of the insulin receptor plays an essential role in insulin signal transduction and insulin action. While the insulin receptor autophosphorylation reaction has been extensively investigated, little is currently known about receptor dephosphorylation, insulin-dependent tyrosine phosphorylation of target proteins, and the mechanism by which insulin mediates its effects on cellular serine/threonine phosphorylation. The objective in this proposal will be to continue characterization of the rapid insulin receptor dephosphorylation reaction which has been demonstrated using the permeabilized rat adipocyte model. The emphasis of this investigation will be on the role of dephosphorylation as a metabolic control mechanism in regulating insulin receptor kinase activity and, thus, insulin action. With the long-term goal of elucidating the interaction between insulin-dependent tyrosine phosphorylation and the cAMP-dependent phosphorylation cascade, the specific aim of this proposal is as follows: Specific Aim - To test the hypothesis that insulin receptor tyrosine kinase activity in adipocytes is regulated in part via a phosphoreceptor dephosphorylation reaction. The experimental objectives are to: 1A) further characterize the rapid dephosphorylation reaction involving the insulin receptor in permeabilized adipocytes, with emphasis on hormonal regulation, inhibitor interactions, and tyrosine residue specificity (using peptide mapping). 1B) to investigate the role of receptor dephosphorylation in insulin resistance by examining this reaction in animal and tissue models (i.e. Streptozotocin-treated, glucocorticoid-treated, and fasted rats, and hyperinsulin-treated adipose tissue). Results of this proposal will contribute to the current knowledge of phosphotyrosine dephosphorylation reactions as they relate to the insulin receptor and insulin action. With increasing evidence that insulin receptor autophosphorylation or substrate phosphorylation are involved in insulin signal transduction, a complete understanding of the entire phosphorylation/dephosphorylation pathway and its possible role as a metabolic control mechanism is essential for an ultimate understanding of normal and abnormal (i.e. insulin resistance) tissue response to insulin.